The present invention relates to a process for the manufacture of oriented hollow articles which are made of thermoplastic material and which possess improved mechanical properties, as well as to a device which is very particularly suitable for carrying out the process.
Numerous processes and devices for blow-molding hollow articles, such as bottles, cans or barrels, of thermoplastic material are already known.
More recently, it has been found that it was possible to substantially improve the mechanical properties of these articles by a judicious choice of the manufacturing conditions so as to induce preferential orientation of the macromolecules in these articles. It has in fact been noted that, for each type of thermoplastic material, a temperature range generally exists for which stretching of these materials, for example under the effect of expansion due to blowing, leads to molecular orientation which causes an improvement in the mechanical properties.
The manufacture of oriented hollow articles made of thermoplastic material, however, necessitates an additional operation compared with the conventional processes, because it is advisable to adjust the temperature of the plastic before carrying out the blow-molding operation. In general, during the manufacture of oriented hollow articles by the technique of blow-molding, a preform is produced in a first step and then, after a second step involving the thermal conditioning of the preform, the latter is transferred to a blow-mold where it is given its final shape while it is at a temperature suitable for promoting its orientation.
According to a first type of process, the preform is produced by injection molding and then, after thermal conditioning in the injection mold and/or outside the latter, the preform is transferred to the blow-mold. However, injection molding of the preform, which is a slow operation, seriously limits the productivity of the installations which apply this technique.
According to a second type of process, the preform is produced from a length of tubular parison which is passed over a support and reheated, while one end of the parison is plastically deformed and sealed by means of a punch which cooperates with the support so as to close the base of the preform. This technique requires that there be discontinuity between the extrusion of the parison and the molding of the preform. Because of this, the substantial amount of heat possessed by the extruded thermoplastic material is lost. Moreover, the reheating process and the molding of the base are also slow operations.
According to a third type of process, which permits the highest rates of production and possesses the greatest economy of means, the preform is produced by blow-molding portions of a tubular parison which is extruded continuously using preform molds. The preforms, after thermal conditioning in the preform molds and/or outside the latter, are introduced into suitable blow-molds. Although this technique possesses the additional advantage of being continuous, it has, however, not hitherto been possible to master it completely satisfactorily, for reasons to be explained below.
During the manufacture of oriented hollow articles, it is advisable, in order to achieve optimum improvement in the mechanical properties, to produce preforms having a diameter which is as small as possible, that is to say as close as possible to the diameter of the starting tubular parison, and having a length which is substantially less than those of the hollow articles desired. In fact, the preform is generally produced under temperature conditions which do not lead to molecular orientation (extrusion temperature). It is consequently advisable to provide for maximum stretching of the preform during the final blow-molding operation which is carried out on it under conditions which promote orientation. It can thus be seen that, by respecting the dual condition given above, there can be produced bi-oriented hollow articles, i.e. hollow articles which are oriented both in the radial direction and in the longitudinal direction, which possess a maximum degree of orientation, and consequently the best mechanical properties.
When the preforms are produced by pre-blowing, they possess a base flash which has to be removed before carrying out the final blow-molding operation in order to prevent any malformation of the bases of the hollow articles produced.
In order to remove this flash, preforms possessing a flat base are produced so that the base flash can be cut off easily by means of a flat knife which follows a rectilinear, or straight line, trajectory against the base of the preform mold.
This method of working leads to preforms whose base part is of irregular shape, because of the pinching effected on the parison and because it is cut along a straight line during the deflashing operation. Because of this, it is found that, during the subsequent final blow-molding operation, this part does not stretch uniformly. As a result of this, the bases of the oriented hollow articles thus produced possess zones of weakness which frequently render the improvement in the mechanical properties imparted by the molecular orientation somewhat illusory.
It has thus not been possible hitherto to develop a process for the manufacture of oriented hollow articles starting from preforms produced by pre-blowing which do not possess zones of weakness in the base part.